organ systems: carbohydrate and protein digestion, pancreatic secretion

[image courtesy of erica newon zelfand]

this unit focused on pancreatic secretions and their role in carbohydrate and protein digestion in the intestine. each day, the pancreas secretes 1L of pancreatic enzymes, among which are enzymes that break down carbohydrates into monosaccharides, enzymes that break down proteins into peptides, lipases which break down triacylglycerols into fatty acids. these digestive enzymes are secreted by the "acinar" cells of the pancreatic ducts, while water and electrolytes such as bicarbonate are secreted by duct cells (a similar strategy to salivary and gastric secretion). the bicarbonate from duct cells comes from carbonic acid, which is formed in duct cells by carbonic anhydrase. bicarbonate is transported into the duct lumen in exchange for a chloride ion by the CFTR transporter. the acinar cells can be stimulated to produce more secretion either directly by vagal stimulation or by different neuropeptides such as CCK, GRP, SubP, VIP. duct cells are stimulated to increase water and bicarbonate production mainly by secretin (which functions to reduce acidity in the intestine and as such also inhibits gastric secretion and emptying).

as with gastric secretion, pancreatic secretions can be divided into three phases (refer to comparison chart), cephalic, gastric, and intestinal. in the cephalic phase, thoughts or sensation of food causes increase of pancreatic secretion via vagal stimulation. in the gastric phase, gastric distention triggers vagal stimulation of pancreatic secretion. the most important phase for pancreatic secretion is the intestinal phase, where intestinal distention, high acid levels, or other digestive products trigger hormonal release that modify pancreatic secretion. CCK is released from I cells in response to fat and proteins, which stimulates acinar cells to increase enzymatic secretion. secretin is released from S cells in response to acid and fat, which stimulates bicarbonate and water secretion from pancreatic duct cells.

carbohydrate digestion, already initiated with salivary alpha amylase, continues with the secretion of pancreatic alpha amylase into the intestinal lumen. polysaccharides are cleaved into smaller di and tri saccharides, and then further digested by the brush border enzymes (see the biochem chapter for a much more detailed description). different sugars are absorbed into the enterocyte and bloodstream by different transporters: for example, glucose and galactose are absorbed into the enterocyte by SGLT1, a sodium co-transporter, while fructose is facilitatively absorbed by GLUT 5. the pancreas also secretes enzymes that digest proteins, all of which are activated by enterokinase, which activates trypsinogen into trypsin, which activates the other zymogens procarboxypeptidase, chymotrypsinogen, proelastase (see the biochem protein digestion chapter).

an interesting note about the role of tight junctions: most absorption of nutrients occurs through the membrane of the enterocyte, or transcellularly. however, in the case of macromolecules that are too large to be absorbed transcellularly, paracellular transport can occur via modulation of the tight junctions between enterocytes, which can occur reversibly via the molecule zonulin. paracellular transport can also occur in "leaky gut", hyperpermeability of the tight junctions, caused by excess glucose levels, alcohol abuse, NSAID/steroidal use,food allergies, crohn's disease. there is a distinct but underexplored relationship between tight junctino permeability and autoimmune disease; people with dysregulation of tight junction permeability have a higher susceptibility for autoimmune diseases.

questions
1. enzymes digest carbohydrates, fats, proteins by what type of reaction?
2. what is SGLT1 and what does it do?

describe the absorption of these substances from the intestinal lumen into the enterocyte, and from there into the blood stream:
3. glucose
4. lactose
5. fructose
6. glycogen
7. sodium

tight junctions...
8. describe the absorptive pathway from the intestinal lumen to the mucosal capillary.
9. what is a function of tight junctions in intestinal epithelium that is not commonly discussed?
10. what is the relationship of tight junctions and the immune system?
11. what are tight junctions made of?
12. what is "leaky gut" and what are some factors that cause it?
13. what is zonulin and what does it do?
14. what is lactulose and how is related to leaky gut?

pancreas...
15. where is the pancreas?
16. how much liquid does the pancreas secrete per day?
17. what do the acinar and duct cells secrete?
18. what are the electrolytes that are secreted by duct cells?
19. describe the secretion of bicarbonate by duct cells.
20. what are the enzymes that are secreted by the pancreas that digest proteins?
21. how are the protein digestion enzymes activated?
22. what does pancreatic amylase do?
23. what are the pancreatic enzymes that digest lipids?
24. what are the different ways in which pancreatic acinar cells are stimulated?
25. what are the actions of secretin?

pancreatic phases...
26. what are the three phases of pancreatic secretion?
27. describe the cephalic phase of pancreatic secretion.
28. describe the gastric phase of pancreatic secretion.
29. describe the intestinal phase of pancreatic secretion.
30. describe the secretion and actions of CCK.
31. describe the secretion and actions of secretin.
32. how does enzymatic secretion adapt to different dietary compositions?

carbohydrate digestion...
33. what are the approximate proportions of the different types of carbohydrates ingested?
34. pancreatic and salivary amylase digests starch into...
35. how are di and trisaccharides digested into monosaccharides?
36. what are the two ways that glucose and galactose can be transported into the intestinal epithelium?
37. compare the absorption of glucose and fructose.
38. describe the negative feedback that can occur in carbohydrate digestion.
39. how can malfunctioning carbohydrate digestion cause diarrhea?
40. what is the hydrogen breath test?
41. what effect does celiac disease has on the intestinal lining?

protein digestion and absorption...
42. how are proteins digested in the stomach?
43. how are proteins digested in the intestine?
44. how are polypeptides digested and absorbed in the intestine?

pancreatic tests...
45. what is the stool chymotrypsin test?
46. what is the pancreatic elastase test and why might it be more reliable than the stool chymotrypsin test?


answers
1. hydrolysis reactions.
2. it is a sodium / glucose co-transporter, facilitating absorption of glucose, galactose

3. glucose is transported into the enterocyte via SGLT1, then into the blood via GLUT2.
4. lactose is broken down into glucose and galactose via the brush border enzyme lactase, and glucose and galactose are absorbed and transported in the same way as question 3.
5. fructose is transported into enterocytes via GLUT5, and absorbed into blood via GLUT2.
6. glycogen is broken down into oligsaccharides and alpha limit dextrins by alpha amylase, and then broken down further into monosaccharides by brush border enzymes and absorbed the same way as in question 3.
7. sodium is transported along with glucose or galactose via the SGLT1 co-transporter.

8. intestinal lumen -> unstirred layer of fluid -> glycocalxces on microvilli of enterocytes -> cell membranes and cytoplasm -> basement membrane -> capillary
9. tight junctions can facilitate absorption of macromolecules, nutrients that are too big to be absorbed directly into enterocytes.
10. when tight junction regulation of macromolecule absorption is dysregulated, this can sometimes lead to intestinal and extraintestinal autoimmune disorders.
11. occludins, claudin family of proteins, junctional adhesion molecules.
12. hyperpermeability of the intestinal epithelium. caused by alcohol abuse, high sugar intake, food allergies, NSAIDS/steroid drugs, celiac disease and crohn's disease.
13. a molecules that facilitates the permeability of tight junctions and as such is involved in the absorption of fluid and macromolecules across the intestinal barrier, and also protect the intestine from being colonized by microorganisms.
14. a macromolecule that can only be absorbed paracellularly; its presence in the blood as compared to mannose (which is absorbed transcellularly) is a good indicator of leaky gut.

15. the body of the pancreas lies deep to the stomach and the tail extends to the spleen.
16. 1L per day
17. acinar cells secrete enzymes, duct cells secrete water and electrolytes.
18. bicarbonate and sodium.
19. bicarbonate is formed from carbonic acid which is formed by carbonic anhydrase and CO2. H+ is absorbed into blood, and bicarbonate is transported into the lumen by the transmembrane regulator CFTR (cystic fibrosis transmembrane regulator) which exchanges a chloride ion for a bicarbonate ion.
20. trypsinogen, proelastase, chymotrypsinogen, procarboxypeptidase.
21. trypsinogen is activated by the brush border protease enterokinase, forming trypsin, which activates the other enzymes.
22. an endoglucosidase that hydrolyzes carbohydrates into di and tri-saccharides.
23. pancreatic lipase, cholesterol esterase, phospholipase.
24. directly via the vagus nerve, or indirectly via stimulation of the vagus nerve by CCK, VIP, GRP, SubP.
25. stimulates duct cells to release bicarbonate, inhibits gastric secretion and emptying- overall effect is to raise pH.

26. cephalic, gastric, intestinal
27. stimulated by thoughts or sensation of food, vagus nerve stimulates pancreas secretion.
28. gastric distention causes vagal stimulation, which causes pancreas secretion.
29. the most important phase for pancreatic secretion; digestive products or low pH trigger release of hormones that control secretion.
30. CCK is secreted from I cells in the intestinal epithelium in response to proteins or fats. chief among its many effects is to stimulate pancreatic acinar secretion of enzymes.
31. secretin is released from S cells in the intestine in response to acid or fat-- its main effect is to increase water and bicarbonate secretion by pancreatic duct cells, thereby raising the pH of the intestinal lumen. it also decreases gastric motility and emptying.
32. CCK can up or downregulate the protein digesting proteases and the carbohydrate digesting amylases depending on the ratio of protein to carbohydrates in the diet.

33. 50% starch, 20% sucrose, 6% lactose, 1-2% maltose.
34. di and trisaccharides.
35. by the brush border enzymes.
36. transcellularly via the SGLT1 Na+ cotransporter or paracellularly with water with high glucose concentrations (see section on tight junctions)
37. fructose absorption is slower and uses a facilitated transporter rather than a co-transporter. (see question 5)
38. chemo and osmotic receptors sense high glucose levels in the duodenum and jejunum and trigger the vasovagal reflex, which decreases gastric motility and emptying.
39. in the case of lactose intolerance, lactose remains undigested and cause a hyperosmolar intestinal lumen from excess sugar as well as products from bacterial processing of these sugars; causing water to accumulate in the lumen and cause diarrhea.
40. a test which measures amount of hydrogen in breath, which is produced from bacteria that metabolize lactose in the intestine in lactose intolerant people.
41. destroys villi.

42. gastric pepsinogen is secreted and converted to pepsin by the low pH environment of the stomach. it cleaves proteins into smaller polypeptides.
43. in the intestine, proteins are digested by different pancreatic enzymes: trypsin, chymotrypsin, carboxypeptidase, elastase. trypsin activates the other three enzymes, which cleave large polypeptides into small polypeptides.
44. small polypeptides are cleaved further into smaller polypeptides and amino acids, which are then transported into the enterocyte and then absorbed into the blood by carrier proteins.

45. a marker for pancreatic output; normal is >9U/g stool, low is <>200 mcg/g stool, low is <100>
46. another test for pancreatic output, which might be more accurate because elastase is only produced by human pancreatic cells. normal is >200 mcg/g stool, low is <100>

biochem: mark's medical biochem chapter 37- protein digestion

this chapter looked at the digestion, absorption, and transport of proteins. digestion of proteins begins in the stomach, when the zymogen pepsinogen (recall that this is secreted by the parietal cells in the gastric pits) is autocatalytically cleaved to pepsin. pepsin works as an endopeptidase, randomly cleaving peptide bonds within the protein.

in the intestine, protein digestion continues with enzymes secreted from the pancreas, in zymogenic (inactive) form: trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase. trypsinogen is cleaved first by enteropeptidase into trypsin. trypsin both works to digest proteins directly by cleaving peptide bonds adjacent to carboxyl groups contained by arginine or lysine, and more importantly, it activates the other digestive enzymes.

it converts chymotrypsinogen to chymotrypsin, which then cleaves peptide bonds adjacent to carboxyl groups from acidic or hydrophobic amino acids. it converts proelastase to elastase, which then cleaves peptide bonds in elastin as well as those next to carboxyl groups contained by small chain amino acids such as alanine, glycine, and serine. finally, it activates carboxypeptidase from procarboxypeptidase, which acts an exopeptidase; cleaving amino acids one at a time from the outside edges of the protein fragments created by digestion via the other pancreatic enzymes.

in addition to pancreatic enzymes, the intestinal epithelial cells also contain protein digesting enzymes; these include aminopeptidases, which also act as exopeptidases, and intracellular peptidases, which break down protein fragments that have been absorbed by the enterocytes.

the amino acids produced from degradation of proteins are transported into enterocytes via Na+ cotransporters, similar to the glucose / Na+ cotransporters seen in carbohydrate digestion. these transporters are powered by the low Na+ concentration in the cells which is manufactured by the Na+/K+ pumps. thus amino acids are absorbed via secondary active transport- and then they diffuse out of the serosal side via facilitated transport proteins.

the "intracellular amino acid pool" is a measure of how much amino acid there is at any given moment within a cell. this is a dynamic balance between proteins acquired from the diet vs. the degradation of proteins. the "half life" of a protein refers to the point at which 50% of the protein has been degraded; proteins in muscle cells, digestive enzymes, or hemoglobin, are all examples of proteins which have a short half life and therefore a high "turnover" rate.

a few notes about intracellular protein digestion: cells can digest proteins by the process of "autophagy", in which proteins in extracellular vesicles fuse with lysosomes, which contain proteases which degrade the proteins into amino acids, which then are absorbed into the cytoplasm. another method of protein digestion is through the ubiquitin / proteasome pathway, in which ubiquitin is attached to proteins, "tagging" them for digestion via proteasomes, which are large barrel shaped proteins with multiple internal proteolytic sites.

questions
1. what form are the protein digesting enzymes secreted in?
2. where are parietal and chief cells located and what do they secrete?
3. how is pepsinogen activated?
4. describe the action of pepsin on proteins.
5. what are the zymogens that are secreted by the pancreas into the intestine?
6. describe trypsinogen's role in protein digestion in the intestine.

7. how is trypsinogen activated?
8. how does trypsin directly digest proteins?
9. describe the digestive action of chymotrypsin on proteins.
10. describe the digestive action of elastase on proteins.
11. describe the action of carboxypeptidase on proteins.
12. what is the difference between carboxypeptidase A and B?
13. where are aminopeptidases located and what do they do?
14. what are intracellular peptidases?

15. describe the transport of amino acids from the intestinal lumen into enterocytes.
16. describe the transport of amino acids from the enterocyte to the portal vein.
17. describe the diversity of amino acid transport proteins on the apical side of enterocytes.
18. how do amino acids get transported into cells of peripheral tissues?
19. in what way does transport and absorption of amino acids into peripheral tissues differ from that of carbohydrates?

20. what is the half life of a protein?
21. what are some examples of proteins that undergo extensive synthesis and degradation in the body?
22. how much of the cells lining the intestinal wall are replaced each day?
23. what percentage of proteins that are absorbed from the intestines are excreted?

24. what is autophagy?
25. which enzymes in lysosomes aid in protein digestion?
26. what is the ubiquitin-proteasoms pathway?
27. how does ubiquitin "tag" proteins?
28. what is a proteasome?

answers
1. zymogenic: an inactive, larger form of the enzyme that is activated by proteolytic cleavage once in the digestive tract.
2. in the gastric pits / epithelium of the stomach. parietal cells secrete HCl and chief cells secrete pepsinogen.
3. the acidity in the stomach allows pepsinogen to be cleaved to its active form, pepsin.
4. pepsin acts as an endopeptidase, cleaving peptide bonds at random intervals within the denatured protein.
5. trypsinogen, pepsinogen, proelastase, procarboxypeptidase.
6. trypsin (the activated form of trypsinogen) catalyzes the activation of the other pancreatic enzymes to their active forms, as well as directly aiding in the digestion of proteins.

7. through enteropeptidase.
8. trypsin cleaves peptide bonds adjacent to carboxyl groups that are provided by lysine or arginine.
9. chymotrypsin cleaves peptide bonds next to residues that contain hydrophobic or acidic amino acids.
10. elastase cleaves peptide bonds within elastase as well as bonds next to residues with small side chains (alanine, glycine, serine)
11. carboxypeptidase acts an exopeptidase, removing amino acids one at a time from the carboxyl end, from the smaller peptides resulting from breakdown of the other pancreatic enzymes mentioned above.
12. A preferentially cleaves hydrophobic amino acids while B preferentially cleaves basic amino acids.
13. they are located on the epithelial wall of the intestine and act as exopeptidases, removing one amino acid at a time.
14. the enzymes within cells that break down peptides which are absorbed.

15. a Na+/K+ pump in the enterocyte creates a low concentration of Na+ in the enterocyte. The resulting influx of Na+ is coupled with amino acid transport, this is called secondary active transport.
16. the amino acids in the enterocytes are transported into the portal vein via "facilitated transporters"
17. there are at least 6 such transport proteins which have overlapping specificities for different types of amino acids.
18. mainly through Na+ cotransporters.
19. amino acids are transported into cells mainly by Na+ cotransporters wheras in peripheral tissues carbohydrates are transported by facilitated transporters (recall the GLUT transporters). in the intestine and renal cells the absorption of both amino acids and carbohydrates are Na+ coupled.

20. the point at which 50% of the protein in a cell has been degraded.
21. hemoglobin, muscle proteins, digestive enzymes.
22. roughly 1/4th.
23. roughly 6%.

24. the process by which cells digest proteins using lysosomal enzymes.
25. the cathepin family of proteases.
26. a method of intracellular protein digestion using ubiquitin tagging and degradation via proteosomes.
27. by covalently binding to the epsilon-amino group of lysine residues.
28. a cylindrical 20S protein complex with multiple internal proteolytic sites.